Is sample conditioning critical with NIR?
Yes. Water in the sample will cause major problems, including total absorbency of all the IR spectra, as water absorbs at over 10 times more than any hydrocarbon. There will also be a back scattering interaction of the water with the IR energy. ASTM E1252-88 states that all water must be removed from the samples before measurement. Water can be removed using either molecular sieves or swirl clean filters with hydrophilic membranes. However, neither are 100% effective and both require maintenance to regenerate or replace. Water vapor in the atmosphere surrounding the NIR spectrometer is also important. This requires an “optical purge” to remove water and hydrocarbon vapors from the sample cell. A molecular sieve may be used. The hydrocarbon dew point of the purge air needs to be below -48 deg C, and, of course, it needs to be oil free. NMR does not have any similar requirements for its sampling probe area. Particulates are another problem for NIR, as the sample cell is only 2 500 micron
